Slide-bars of this type are provided along at least two opposing longitudinal sides of the levelling plates. As a rule, a second rail, of angular cross-section, runs above each guide rail and rests with the narrow side of one limb against the side surface of the levelling plate, so that the other limb projects upwards, or better still downwards, rather in the form of a hook. The carriage of the measuring and/or tracing device bears a vertical column to which is secured a transverse sliding unit which can be moved in a vertical direction and which holds a cross arm moving horizontally and parallel to the plane surface of the levelling plate and thus at right angles to the path of the slide-bar.
The purpose of the upper angle-shaped rail is to support the carriage mainly in a direction approximately at right angles to the side surface of the levelling plate. To achieve this, small rollers on the carriage engage inside the projecting hook-shaped limb. The guide rail running a short distance below it serves two purposes. Its horizontal guide surface acts as a horizontal supporting surface on which the carriage is borne in a direction at right angles to the plane surface of the levelling plate, running on guide rollers. In addition, this horizontal guide surface determines the horizontal direction of travel of the carriage. The guide rail must therefore be accurately aligned so that the horizontal guide surface runs parallel to the plane surface of the levelling plate, at least when regarded in the direction of travel of the carriage. Any deviation from this horizontal path results in the carriage running uphill or downhill on the horizontal guide surface, leading to an increase or decrease in the vertical distance between the plane surface of the levelling plate and a fixed point on the cross arm when the carriage moves along the horizontal guide surface. Accurate measuring and/or tracing operations would then be impossible. The guide rail must therefore be aligned in the vertical plane with sufficient accuracy to ensure that, when the carriage moves along the horizontal guide surface, the cross arm of the device always runs and remains at the same vertical distance from the plane surface of the levelling plate.
The longitudinal guide surface determines the directional path when the carriage is operated. In this case, it is important that, as regards its linear path in a horizontal plane running parallel to the plane surface, the longitudinal guide surface be so aligned that it runs exactly parallel to a linear reference line, such as, for example, the lateral edge of the levelling plate, if the latter has been very accurately machined, or an imaginary linear line in the plane of the plane surface. Deviations would mean that, when the carriage moves along the guide rail, the horizontal distance between the linear reference line and a fixed point on the cross arm, measured in the plane of the plane surface and at right angles to the linear reference line, would decrease or increase instead of remaining constant. It would not then be possible to effect accurate measuring and/or tracing work of a parallel nature within the plane of the plane surface and at right angles to the direction of the cross arm. Any inaccuracy in the alignment of the guide rails, both of the horizontal guide surface on the one hand and of the longitudinal guide surface on the other hand, would thus give rise to relatively large measurement discrepancies, especially in the case of very long guide rails, which can easily be up to 6 meters in length, and these errors can, on the one hand, be in a direction perpendicular to the plane surface and, on the other hand, within the plane of the plane surface and thus at right angles to a linear reference line running at right angles to the direction of the cross arm.
With existing forms of guide rails, such discrepancies cannot be avoided, even if the guide rail is very accurately adjusted. The guide rail consists of a bar of very large cross-section with a continuous rear supporting surface by means of which the bar can be secured to the side surface of the levelling plate. The supporting surface runs at right angles to the horizontal guide surface and parallel to the longitudinal guide surface. If, with the aid of a spirit level or similar instrument, the guide rail is aligned in such a way that its horizontal guide surface runs exactly horizontally in a linear direction, this does not, as regards the longitudinal guide surface, ensure accurate linearity parallel to an imaginary linear reference line such as the lateral edge of the levelling plate; this is because, as a rule, the side surface of the levelling plate, regarded longitudinally, has surface irregularities, especially shape defects, and thus does not run exactly parallel to an imaginary linear reference line. Because the guide rail is also secured by its supporting surface to the side surface of the levelling plate in the rear sector of the longitudinal guide surface, these discrepancies in the parallelism of the side surface are transmitted to the longitudinal guide surface.